Japanese prosecutors are to investigate 10 immigration officials over a Ghanaian man's mysterious airport death during an attempt to deport him from the country, police said Tuesday.Police have sent files to prosecutors on their investigation into whether the immigration officials used excessive force against the man, a rare move against Japanese civil servants that could lead to indictments.

Abubakar Awudu Suraj, 45, died at Tokyo's Narita airport in March, as the Japanese immigration officials tried to deport him, escorting the restrained man onto an aircraft bound for Cairo.

"The immigration officials restrained the man who had wildly resisted," said a police spokesman without elaborating. "The cause of his death remains undetermined even after a legal autopsy."

Suraj's Japanese widow has filed a complaint with police, challenging authorities to explain how her husband died as he was being deported for illegally staying in Japan.

Rights activists believe he was gagged with a towel, recalling a similar but non-fatal case in 2004 when a female Vietnamese deportee was handcuffed, had her mouth sealed with tape and was rolled up in blankets.

Suraj's death has triggered protests among immigrants and rights activists in Japan, with scores of foreigners in an immigration detention centre launching a temporary hunger strike in May.

Japan keeps a tight control on immigration and last year, despite its generous overseas aid for refugees, granted political asylum to just 30 people.

Human rights activists, lawyers and migrant communities have complained for years about the harsh treatment meted out by immigration officials and about conditions at immigration detention centres